1. Field of the Invention
The present invention relates to an exhaust gas purification apparatus for an internal combustion engine.
2. Description of the Related Art
An automobile includes, for example, a monolith type catalyst provided in an exhaust pipe to purify and discharge an exhaust gas from an engine to the atmosphere. This catalyst purifies the exhaust gas by oxidizing HC and CO in the exhaust gas or reducing NOx therein.
Incidentally, when an exhaust gas is purified only by the aforesaid catalyst, if the exhaust gas is at a low temperature as in such a case, for example, that an engine is started at a low temperature, the catalyst is not activated, and thus a drawback arises in that uncombusted hydrocarbon (HC) passes through the catalyst and discharged to the atmosphere.
To cope with this drawback, Japanese Patent Unexamined Publication Nos. 3-141816, 4-17710 and the like disclose an exhaust gas purification apparatus having an HC absorbent disposed on the upstream side of a catalyst provided in the exhaust pipe of an engine to capture uncombusted hydrocarbon (HC) exhausted from the engine when it starts at a low temperature.
Incidentally, in Japanese Patent Unexamined Publication No. 3-141816, when an exhaust gas is at a temperature less than 200.degree. C., HC is absorbed by the absorbent, whereas when the exhaust gas is at a temperature exceeding 200.degree. C., absorbed HC desorbs therefrom on the contrary. Thus, a problem arises in that when a catalyst disposed on the downstream side of the absorbent is not activated, the HC passes through the catalyst and is discharged to the atmosphere. Therefore, there are provided a bypass pipe for bypassing the absorbent and a change-over valve provided at the inlet of the bypass pipe. When an exhaust gas is at a temperature of 250.degree. C. or lower, the bypass pipe is closed to conduct the exhaust gas to the absorbent to enable the same to be absorbed by the absorbent, and when the exhaust gas is at a temperature exceeding 250.degree. C., the bypass pipe is opened to check the flow of the exhaust gas to the absorbent and prevent the desorption of the HC absorbed by the absorbent. Further, when the exhaust gas exceeds 300.degree. C., the bypass pipe is closed to conduct the exhaust gas to the absorbent to desorb the HC and the desorbed HC is purified by the catalyst located on the downstream side.
As described above, a problem arises in that the change-over valve for opening/closing the bypass pipe must be controlled in a narrow temperature range from 250.degree. C. to 300.degree. C.
Further, when the temperature of the absorbent is increased by the heat conduction through the exhaust pipe in the state that the bypass pipe is closed and HC is desorbed, if the catalyst located on the downstream side is not activated, a problem arises in that the HC passes through the catalyst and discharged to the atmosphere in a large amount.